1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to apparatus and method for determining a channel estimate for use in reconstructing data symbols transmitted over a channel.
2. Related Art and other Considerations
A wireless telecommunications unit typically includes both a transmitter and receiver for communicating with other wireless telecommunications units over a communication link. For wireless communications, the communication link is typically over an air interface (e.g., radio frequency interface). As used herein, a “wireless telecommunications unit” with its “wireless telecommunications receiver” can be included in a network node (e.g., a radio access network node such as a base station node, also called Node-B) or a terminal. Such “terminals” include mobile terminals such as user equipment units (UEs), which have also been called mobile stations, and include by way of example mobile telephones (“cellular” telephones), laptops with mobile termination. Thus, terminals can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the terminals can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
As shown simply in FIG. 32, a wireless telecommunications system includes a transmitting antenna 2300T and a receiving antenna 2300R. Channel 2302 describes the relation between the transmitting antenna 2300T and the receiving antenna 2300R, including the wireless interface. A signal, typically modulated into pulses, is transmitted over channel 2302 from transmitting antenna 2300T to receiving antenna 2300R. The signal can comprise a “symbol” or a string of series of symbols, depicted as “m” in FIG. 32. The signal can carry user data and/or certain control data (e.g., a pilot bit or pilot sequence). The signal m as transmitted by the transmitting antenna 2300T is convoluted with a channel impulse response h of the channel, so that the received signal at the receiving antenna 2300R is m*h (e.g., m convoluted with h). The received signal m*h is applied to base band processing functionality 2304 of the receiver where the received signal undergoes radio frequency processing. The data portions of the received signal are applied to a detector 2306, which may be, for example, a demodulator such as a RAKE receiver.
Most modern detectors attempt to recover a symbol estimate {circumflex over (m)} from the received signal m*h. To do so, most sophisticated detectors expect to receive a “channel estimate” for use in modeling the channel over which the signal was transmitted. The accuracy of this channel estimate influences the accuracy and performance of the detector in estimating the actual symbol received over the channel.
The modeling of the channel (which is necessary for most detectors) is facilitated by the control data, often in the form of a pilot bit or pilot sequence, which is transmitted by the transmitter. The control data, hereinafter referenced as “pilot data” for simplicity, is of a known or recognizable format or pattern. The pilot data is typically transmitted periodically by the transmitter source, and thus receipt of repetitions of the pilot data can be expected at the receiver at successive intervals. In view of factors such as relative motion of the transmitter and receiver, the successive intervals are not necessarily constant. The pilot data can be transmitted simultaneously with, or otherwise interspersed with, the user data.
In order to utilize the pilot data, wireless receivers typically include both a searcher and a channel estimator, such as searcher 2308 and channel estimator 2310 shown in FIG. 32. For control data, the received signal m*h is applied to the searcher 2308, which determines a time of arrival (TOA). The time of arrival is then applied to the channel estimator 2310, which uses the time of arrival to determine the channel estimate ĥ and then provides the channel estimate ĥ to the detector 2306. Using the channel estimate ĥ, the detector develops its estimate of the symbol, e.g., {circumflex over (m)}.
The receiver may receive the an original signal (e.g., short pulse signal) from the transmitter source through open space over a single, direct propagation path. Alternatively, in another environment having obstacles or other surfaces, the receiver may receive the same original signal over multiple propagation paths. In the multiple path case, the received signal appears at the receiver as a stream of pulses, each pulse having a different time delay in view of the corresponding propagation multipath over which the signal travelled, as well as possibly different amplitude and phase.
Multipaths are created in a mobile radio channel by reflection of the signal from obstacles in the environment such as buildings, trees, cars, people, etc. Moreover, the mobile radio channel is dynamic in the sense that it is time varying because of relative motion affecting structures that create the multipaths, or due to movement of structures and objects in the surroundings (even if the transmitter and receiver are fixed). For a signal transmitted over a time varying multipath channel, the received corresponding multiple paths vary in time, location, attenuation, and phase.
Some wireless telecommunications receivers capitalize upon the existence of the multipaths in order to achieve various advantages. Such receivers typically operate on the baseband signal to search for and identify the strongest multipaths along with their corresponding time delays. The receiver has a filter which operates on a power delay profile of the signal. The power delay profile can be conceptualized as a time-averaged refinement or other derivation of the channel impulse response. The searcher attempts to locate peaks in the power delay profile, each peak corresponding to arrival of a wavefront of the signal from a respective multipath. In many searchers the peaks also correspond to a channel tap of the filter.
A channel estimate ĥ as applied to the detector therefore comprises a set of both time of arrivals (TOA) and complex channel coefficients, each pair of TOA and channel coefficients being associated with one of the arriving wavefronts. In other words, each arriving wavefront has a pair of members in the set, e.g., a TOA and a channel coefficient. The channel coefficients thus actually form a channel impulse response vector, so that the terms “channel coefficient” and “channel coefficients” as used hereinafter should be understood to refer to a channel impulse response vector. If there is only one wavefront, there is only one TOA and one channel coefficient in the set (one channel coefficient in the channel impulse response vector). But for plural arriving wavefronts, there are a corresponding plurality of TOAs and channel coefficients. Ideally, the channel estimate ĥ should provide as good an estimate of the channel impulse response as possible, thereby increasing performance of the detector as the detector makes its estimate {circumflex over (m)} of the transmitted symbol m.
The channel estimate is then supplied to the detector, such as RAKE type of demodulators. A RAKE demodulator typically allocates a number of parallel demodulators (called RAKE fingers) to the strongest multipath components of the received multipath signal as determined by the multipath search processor. In a wideband code division multiple access (WCDMA) radio access network, the outputs of each of the RAKE fingers are diversity-combined after corresponding delay compensation to generate a “best” demodulated signal that considerably improves the quality and reliability of the radio communications system.
Conventionally, wireless telecommunications receivers first use their searchers to ascertain time of arrival of a wavefront. Subsequently, after the time of arrival has been determined by the searcher, the channel estimator utilizes the time of arrival to calculate a channel coefficient, which expresses both amplitude and phase of the signal.
Some wireless telecommunications units have more than one antenna for receiving a same signal. In the prior art, the searcher attempts to locate peaks in the power delay profile for each antenna separately. In other words, for each antenna the searcher works more or less independently. See, for example, U.S. Patent Publication US 2002/0048306, which is incorporated herein by reference. As such, the prior art searchers are essentially one dimensional.
As indicated above, the performance of a wireless receiver is considerably dependent upon the accuracy of the peak determination, i.e., time of arrival determination, performed by the searcher. The better the peak determination of the searcher, the better will be the overall performance of the receiver (e.g., less error rate). But in many instances it may be difficult for a searcher to find an actual peak in a power delay profile. As mentioned previously, in many searcher algorithms the peak corresponds to a channel tap. With such difficulty there is considerable risk of incorrectly choosing a peak. Moreover, it can then be difficult to estimate the actual channel tap value. Channels with low signal to noise ratios (SINRS) are particularly susceptible to these difficulties.
What is needed, therefore, and an object of the present invention, is provision of apparatus and method for providing an improved channel estimate for a wireless telecommunications receiver.